The present invention relates to a stage apparatus, a projection optical apparatus and an exposure method. More particularly, the present invention relates to a stage apparatus including a material support capable of positioning a material piece carried thereon with at least three degrees of freedom, a projection optical apparatus including such a stage apparatus used as a positioning stage for a material piece, and an exposure method for transferring a pattern formed on a mask through a projection optical system by exposure to a photosensitive substrate loaded on a movable stage. One representative example of the above-mentioned projection optical apparatus is a projection exposure apparatus, such as a stepper, for use in a photolithographic process forming a part of the fabrication process of semiconductor devices, liquid crystal displays or other products. Another example is lens inspection equipment for measuring various aberrations associated with a projection optical system (or projection lens) equipped for such a projection exposure apparatus.
In photolithographic process for fabricating semiconductor devices, liquid crystal displays or other products, there have been used various projection exposure machines in which an exposure light beam is used to illuminate a mask or a reticle (a generic term xe2x80x9creticlexe2x80x9d is used to mean either hereinafter) so as to transfer a pattern formed on the reticle through a projection optical system onto a photosensitive substrate. Further, in order to measure aberrations associated with a projection optical system equipped for such a projection exposure machine, there has been used lens inspection equipment in which various aberrations with the projection optical system are measured by making a projection exposure so as to transfer an image of a reticle pattern onto a photosensitive substrate and analyzing the exposure result (or the resulting pattern image formed on the photosensitive substrate.)
With these machines and equipment, a stage apparatus is typically used for positioning a material piece or substrate having a photosensitive layer coated on its surface. The substrate may typically comprise a silicon wafer or a glass plate (referred to simply as a xe2x80x9cwaferxe2x80x9d hereinafter). One common stage apparatus comprises an XY-stage capable of two-dimensional movement in a plane (typically, an XY-plane) and a material support (or Z-stage) for supporting a material piece or wafer thereon, the material support being mounted on the XY-stage and capable of displacing in the Z-direction perpendicular to the XY-plane.
By way of example, consider a demagnification projection exposure apparatus of the step-and-repeat type (commonly referred to as a xe2x80x9cstepperxe2x80x9d). In an exposure apparatus of this type, after the exposure of a shot-area, the wafer is moved or stepped to the next shot-area and the process is repeated until the entire surface of the wafer (or every shot-area on the wafer) has been processed to have reticle patterns transferred onto it. The stepping movement of the wafer is produced by the two-dimensional movement or translation of the XY-stage supporting that wafer.
With recent progress in the microminiaturization of circuit patterns exposure apparatuses are required to provide higher performance including an improved overlay accuracy as well as a higher resolution. Because a typical demagnification exposure apparatus uses a projection lens having a large numerical aperture (NA), its depth of focus is so small that a certain, appropriate mechanism must be provided for bringing with precision the surface of the wafer to a position at which it will be coincident with the image plane of the reticle pattern defined by the projection lens (such a mechanism may be, for example, an auto-focusing mechanism or a leveling mechanism). Also, it is desired to measure minute irregularities in the nominally flat surface of a wafer, or variation in height of the surface of a wafer placed on the stage among different points on the wafer surface. That is, wafer-flatness measurement is desired. Further, it is also desired to measure with accuracy various aberrations (such as, variation in depth of focus, curvature of image plane, tilt of image plane and others) associated with a projection optical system.
Wafer-flatness measurement, as well as the measurement of aberrations with the projection lens by lens inspection equipment, have been typically performed by using the positions of the moving XY-stage as the reference for the measurement. Specifically, in the case of wafer-flatness measurement, the XY-stage is driven so as to sequentially move the wafer into a number of predetermined measurement points arranged in a rectangular array or matrix, and the height (or the position in the Z-direction) of the surface of the wafer is measured at each measurement point. The Z-direction is perpendicular to the plane in which the XY-stage moves and is usually defined as the direction along the optical axis of the projection optical system.
Unfortunately, the surface of the wafer or material piece carried by the XY-stage may rise and sink (or displace in the Z-direction) while the XY-stage is driven (or more exactly an X-stage and/or a Y-stage, together composing the XY-stage, are driven) to move the wafer in the XY-plane. In the case where the drive mechanism for the stage comprises a feed screw in threading engagement with the stage and an electric motor for rotating the feed screw, such displacement in the Z-direction may be possibly caused by various factors, including minute irregularities in the nominally flat bearing surface along which the stage is guided, an inevitable play between the feed screw and the stage engaging with each other, some distortion induced in the feed screw, and others.
Therefore, the rise/sink of the stage produced when the stage is moved into different positions, or the displacement characteristic of the stage, often causes errors in the results of the wafer-flatness measurement and the lens-aberration measurement. Further, unlike the yaw of wafers which may be separately measured with ease by a yaw-measuring interferometer, it has been impossible to separately measure the displacement characteristic of the stage (or the rise/sink of the stage), with the result that the wafer-flatness measurement and the lens-aberration measurement have been performed so far, allowing for such errors.
Nevertheless, with the continuing increase in the complexity of present-day integrated circuits (ICs), the measurement errors caused by an undesirable displacement characteristic of the stage are becoming unacceptable, and it is almost certain that such measurement errors are not acceptable for the fabrication of the next generation of 64-megabit dynamic random access memories (DRAMs) any longer.
On the other hand, with respect to exposure apparatuses used in producing, for example, semiconductors, in order to transfer a pattern on a mask through a projection optical system to a photosensitive substrate with high accuracy, it is important to correct an amount of tilt of the photosensitive substrate which is provided on a movable stage, relative to an image plane with respect to the projection optical system. In a conventional exposure apparatus, in order to suppress variations in an amount of tilt of the movable stage during movement thereof, a stage-driving surface on which the movable stage is moved is machined with high precision according to the type of exposure apparatus, thereby flattening the stage-driving surface. However, in the above-mentioned conventional exposure apparatus, there is a disadvantage, such that correction of the amount of tilt of the movable stage is dependent on the degree of precision in machining the stage-driving surface, so that an improvement in accuracy of correction of the amount of tilt of the movable stage is limited. Further, due to the need for machining with high precision, production costs of stages become undesirably high.
In order to obviate such disadvantages, various countermeasures have been attempted. For example, Unexamined Japanese Patent Application Public Disclosure No. 5-283310 discloses an exposure method in which the amount of tilt of the surface of photosensitive substrate relative to the image plane with respect to the projection optical system is corrected in a global manner by a so-called EGL (Enhanced Global Leveling) method. In the EGL method, after a photosensitive substrate to be subjected to exposure is loaded on a stage, the amount of tilt (concave and convex) of the surface of the photosensitive substrate relative to the image plane with respect to the projection optical system is measured at a plurality of measurement sites. The amount of tilt of the entire surface of photosensitive substrate is determined statistically from the respective values of measurements at the plurality of measurement sites, and based on the thus determined amount of tilt, the tilt of the surface of photosensitive substrate is corrected.
However, in the above-mentioned EGL method, measurement of the amount of tilt of the entire surface of photosensitive substrate must be conducted by statistic calculations with respect to each photosensitive substrate after loading, so that a throughput becomes low.
In view of the foregoing, it is an object of the present invention to provide a stage apparatus which may provide an improved measurement accuracy in various measurement processes in which the positions of the moving stage are used as a reference for the measurement.
It is another object of the present invention to provide a stage apparatus which is capable of correcting for the rise/sink of the stage during such a measurement process.
It is a further object of the present invention to provide a stage apparatus which may improve measurement accuracy with the positions of the moving stage being used as the reference for the measurement.
It is a yet further object of the present invention to provide a projection optical apparatus which is capable of performing various measurement processes in which the positions of the moving stage are used as the reference for the measurement, such as the flatness measurement of a substrate, with great precision.
It is a yet further object of the present invention to provide an exposure method in which stages produced at low cost can be used and a high throughput can be achieved.
In accordance with the present invention, there is provided a stage apparatus comprising: a stage; a driver which drives the stage for movement in a direction along a first axis in a predetermined plane; and a first measuring device which measures an amount of displacement of the stage in a direction perpendicular to the predetermined plane, as caused by movement of the stage in the direction along the first axis.
The above-mentioned stage apparatus may further comprise: a second measuring device which measures a coordinate of a position of the stage in the direction along the first axis; and a memory which stores displacement data indicative of the amount of displacement of the stage in the perpendicular direction, as caused by movement of the stage in the direction along the first axis, in association with the coordinate of the position of the stage in the direction along the first axis.
The second measuring device may further measure a coordinate of the position of the stage in a direction along a second axis perpendicular to the first axis in the predetermined plane and the memory may store displacement data of the amount of displacement of the stage in the perpendicular direction, as caused by movement of the stage in the predetermined plane, in association with the coordinate of the position of the stage in the direction along the first axis and the coordinate of the position of the stage in the direction along the second axis.
The amount of displacement of the stage in the perpendicular direction may be derived by an arithmetic operation, based on distance data indicative of a distance in the perpendicular direction between a surface of a plane-parallel surface-mirror placed on the stage and a predetermined reference point as measured while moving the stage with predetermined intervals in the predetermined plane.
The above-mentioned stage apparatus may further comprise an adjuster which adjusts the position of the stage in the perpendicular direction depending on the amount of displacement of the stage in the perpendicular direction, while the stage is moved.
The above-mentioned stage apparatus may further comprise an arithmetic operation system, and when the stage is moved to a position for which the displacement data in the memory contains no displacement data indicative of the amount of displacement of the stage, the arithmetic operation system may derive the amount of displacement of the stage at the position for which the displacement data in the memory contains no displacement data indicative of the amount of displacement of the stage, by performing interpolation on the displacement data in the memory.
In the above-mentioned stage apparatus, the stage may include a material support which is capable of being displaced by a minute amount in the perpendicular direction, the first measuring device may measure the amount of displacement of the material support in the perpendicular direction, as caused by movement of the stage in the predetermined plane, and the second measuring device may further measure a coordinate of a position of the material support in the direction along the first axis and a coordinate of the position of the material support in the direction along the second axis.
In accordance with the present invention, there is also provided a method of moving a stage, comprising the steps of: moving the stage in a predetermined plane; measuring an amount of displacement of the stage in a direction perpendicular to the predetermined plane as caused by movement of the stage in the predetermined plane; and correcting displacement of the stage moved in the predetermined plane, based on the measured amount of displacement of the stage.
Further, in accordance with the present invention, there is provided an exposure apparatus comprising: a stage device which supports a photosensitive substrate placed thereon; a mask having a pattern formed thereon; an illumination device which illuminates the mask with an exposure light beam; and a projection optical system which projects the pattern on the mask to the photosensitive substrate in the presence of the exposure light beam, and the stage device may comprise a stage, a driver which drives the stage for movement in a direction along a first axis in a predetermined plane and a first measuring device which measures an amount of displacement of the stage in a direction perpendicular to the predetermined plane.
The above-mentioned exposure apparatus may further comprise: a second measuring device which measures a coordinate of a position of the stage in the direction along the first axis and a coordinate of the position of the stage in a direction along a second axis perpendicular to the first axis in the predetermined plane; and a memory which stores displacement data indicative of the amount of displacement of the stage in the perpendicular direction, as caused by movement of the stage in the predetermined plane, in association with the coordinate of the position of the stage in the direction along the first axis and the coordinate of the position of the stage in the direction along the second axis.
Still further, in accordance with the present invention, there is provided an exposure method comprising the steps of: moving a stage in a predetermined plane, the stage supporting a photosensitive substrate placed thereon; measuring an amount of displacement of the stage in a direction perpendicular to the predetermined plane as caused by movement of the stage in the predetermined plane; controlling a position of the stage in the perpendicular direction, based on the measured amount of displacement of the stage; and projecting a pattern formed on a mask to the photosensitive substrate.
The above-mentioned exposure method may further comprise a step of measuring coordinates of the position of the stage in the predetermined plane; and storing the measured coordinates of the position of the stage in association with the measured amount of displacement of the stage.
Still further, in accordance with the present invention, there is provided an exposure method for transferring a pattern formed on a mask through a projection optical system by exposure to a photosensitive substrate loaded on a movable stage, the projection optical system having an optical axis, comprising the steps of: measuring an amount of tilt of the movable stage relative to an image plane with respect to the projection optical system at each of a plurality of positions on the movable stage, prior to loading of a photosensitive substrate at a predetermined position on the movable stage; storing the measured amount of tilt of the movable stage; loading the photosensitive substrate at the predetermined position on the movable stage; detecting a position of the photosensitive substrate on the movable stage along the optical axis of the projection optical system; and transferring the pattern on the mask by exposure to the photosensitive substrate, based on the detected position of the photosensitive substrate and the stored amount of tilt of the movable stage.
In the above-mentioned exposure method, the step of measuring the amount of tilt of the movable stage may be conducted at each of a plurality of measurement points which are arranged on the movable stage in the form of intersection points of lines defining segments of a grid, the number of the segments of the grid being defined by the formula mxc3x97n in which each of m and n is an integer.
The above-mentioned exposure method may further comprise a height adjustment step of adjusting the position of the photosensitive substrate along the optical axis of the projection optical system so that the position of the photosensitive substrate along the optical axis of the projection optical system becomes coincident with an optimum image-forming position with respect to the projection optical system and a tilt correction step of correcting the amount of tilt of the movable stage so that the movable stage becomes in parallel to the image plane with respect to the projection optical system. The height adjustment step and the tilt correction step may be conducted at the same time.
In the exposure method of the present invention in which the amount of tilt of the stage is measured and stored as data prior to loading of the photosensitive substrate on the stage and leveling of the stage is conducted after loading of the photosensitive substrate by reading the stored data, it is unnecessary to conduct measurement of the amount of tilt with respect to each photosensitive substrate or each shot area to be subjected to exposure, so that a high throughput can be achieved. Further, differing from conventional techniques, the accuracy in measurement of the amount of tilt of the stage is not dependent on the degree of precision in machining the stage-driving surface, so that cost of production of stages can be reduced to a low level.
Still further, in accordance with the present invention, there is provided an exposure apparatus for transferring a pattern formed on a mask through a projection optical system by exposure to a photosensitive substrate loaded on a stage, the projection optical system having an optical axis, comprising: a measuring device which measures coordinates of a position of the stage in a predetermined plane; a tilt amount measuring device which measures an amount of tilt of the stage relative to an image plane with respect to the projection optical system; a memory which stores the amount of tilt of the stage in association with the coordinates of the position of the stage; a position measuring device which measures a position of a photosensitive substrate loaded on the stage along the optical axis of the projection optical system; and a driver which moves the stage, based on the position of the photosensitive substrate measured by the position measuring device and the amount of tilt of the stage stored in the memory.
In the above-mentioned exposure apparatus, the tilt amount measuring device may measure the amount of tilt of the stage at each of a plurality of predetermined positions to which the stage is moved and the exposure apparatus may further comprise a calculation system which calculates the amount of tilt of the stage at a position different from the plurality of predetermined positions by performing interpolation using the amount of tilt of the stage stored in the memory, when the stage is moved to the position different from the plurality of predetermined positions.
Still further, in accordance with the present invention, there is provided a method for making an exposure apparatus which transfers a pattern formed on a mask through a projection optical system by exposure to a photosensitive substrate loaded on a stage in the presence of illumination light emitted from a light source, comprising: providing a measuring device which measures coordinates of a position of the stage in a predetermined plane; providing a tilt amount measuring device which measures an amount of tilt of the stage relative to an image plane with respect to the projection optical system; providing a memory which stores the amount of tilt of the stage in association with the coordinates of the position of the stage; providing a position measuring device which measures a position of a photosensitive substrate loaded on the stage along the optical axis of the projection optical system; and providing a driver which moves the stage, based on the position of the photosensitive substrate measured by the position measuring device and the amount of tilt of the stage stored in the memory.
Still further, in accordance with the present invention, there is provided a method for making a stage apparatus, comprising the steps of: providing a stage; providing a driver which drives the stage for movement in a first direction in a predetermined plane; and providing a second measuring device which measures an amount of displacement of the stage in a second direction perpendicular to the predetermined plane as caused by movement of the stage in the first direction.
Still further, in accordance with the present invention, there is provided a method for making an exposure apparatus, comprising the steps of: providing a mask having a pattern formed thereon; providing a projection optical system which transfers the pattern on the mask to a photosensitive substrate; providing a stage which supports the photosensitive substrate placed thereon; providing a driver which drives the stage for movement in a first direction in a predetermined plane; and providing a second measuring device which measures an amount of displacement of the stage in a second direction perpendicular to the predetermined plane as caused by movement of the stage in the first direction.
The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of preferred embodiments thereof, reference being made to the accompanying drawings.